Power driven wheel chairs enable persons who must use such conveyances greater freedom of movement. This is particularly apparent in relation to use out of doors. Power driven wheel chairs can go across uneven terrain that a hand propelled wheel chair could not possibly manage.
It is difficult for a power driven wheel chair to maintain all its wheels on the ground when stepping over obstacles. When one of the front wheels of a wheel chair climbs an obstacle, it tends to lift the rear wheel on the same side. When one of the front wheels of a wheel chair descends an obstacle, it tends to lift the rear wheel on the opposite side. These tendencies adversely effect the traction of the rear wheels, and the overall stability of the wheel chair.